The present invention is directed to an improved concrete composition.
With the increasing demand for cementitious products in the building industry, particularly in structures such as buildings, dams, roads, and the like, there is a need for ways of improving the quality of cementitious products without increasing the cost. Due to the trend of increasing manufacturing costs including labor, raw materials and fuels, there is a continuing interest in additive materials which can be mixed with concrete to increase the compressive strength, or which can be used to reduce the amount of cement necessary, while at the same time maintaining the desired compressive strength in the cured concrete.
Concrete is made from a mixture of Portland cement, fine and coarse aggregate usually in the form of sand and gravel or crushed stone, and water. Small amounts of other materials can be added to promote or retard the reactions or to give desired qualities to the cement paste.
Portland cement is essentially a calcium-alumina-magnesia-silicate composition. Addition of the water to the dry mixture commences a series of crystallization reactions by virtue of which the concrete attains the strength necessary for use in building products. Because of the complexity of the chemical reactions leading to concrete formation, researchers have investigated both the reaction conditions and reactants to determine their effect on the cured concrete. These research findings have influenced the establishment of uniform quality standards for cement set forth by the American Society for Testing Materials (ASTM).
Concrete compositions may be modified by the addition of silica and alumina-containing substances known as pozzolana. These materials in themselves have no cementitious or binding quality but react with the lime in the presence of water to form cementitious materials. Pozzolanas are materials such as fly ash, and silica-containing substances such as powdered brick, burnt shale and some of the slags
Fly ash, produced from the burning of pulverized bituminous coal and collected in large quantities by electrostatic precipitation has been suggested as a suitable fine aggregate for concrete. For example, Nelles U.S. Pat. No. 2,250,107 discloses the use of a fly ash having an approximate chemical composition of about 43 percent silica, 25 percent alumina, 15 percent iron oxide, 2 percent calcium oxide and 1 percent magnesia.
During kilning operations to produce commercial grade lime, by driving off CO.sub.2 from quarried limestone, a severe "dusting" problem is encountered. Environmental concerns have led to the extensive use of electrostatic precipitators to collect this lime dust in order to reduce or eliminate discharge into the air. Fine aggregates have been deemed unsuitable for use in concrete if high in magnesia content; in fact, a magnesia content in Portland cement of more than 5 percent is considered deleterious and is not permitted by ASTM specifications. Heretofore the lime dust from lime kilning operations has not been utilized, but has been largely disposed of by land-fill methods such as disposal in the inactive portion of the limestone quarry.